This invention relates to the lubrication of compressors, especially air compressors. More particularly, this invention relates to the lubrication of both the air-end and crankcase-end of compressor equipment with polyol ester-based lubricant compositions to extend operating time between oil changes and reduce required maintenance.
The lubrication of compressors, particularly air compressors, poses what is probably the most difficult of all lubricant applications. A desirable lubricant composition for use in compressors should be liquid over a wide temperature range, should have a low vapor pressure, and be operable over an extended period of time at widely ranging temperatures. The viscosity at high temperatures should be sufficient to provide adequate lubrication, and the viscosity at low temperatures should be low enough to allow start-up of the compressor at subzero temperatures without the need for external heating means. It must, of course, be an effective lubricant.
Up to this point, the requirements of a good compressor lubricant parallel those of a good automotive engine lubricant or a good aircraft engine lubricant. There are, however, additional requirements imposed on compressor lubricants which deserve special consideration.
Unlike automotive or aircraft engine lubricants, compressor lubricants are often brought into direct and intimate contact with the gas being compressed. This contact occurs at elevated temperatures and pressures and is repetitive. Thus, for example, in a rotary screw compressor, the gas being compressed becomes intimately mixed with the lubricant which floods the rotating screw elements. This intimate contact usually occurs at elevated temperatures and pressures. Where the gas being compressed is air, the oxygen content of the air in combination with the high pressure and high temperature presents an oxidizing atmosphere which is much more severe than that normally encountered by lubricants in automotive or aircraft engines.
In both engines and compressors, some of the lubricant is exposed to the "process side" of the equipment and is discharged from the equipment along with the other discharge products following such exposure. Thus, in a piston engine, some of the lubricant forms a film on the walls of the combustion chambers of the engine. This oil film may then be consumed by the combustion and/or discharged with the combustion products. These discharge products are almost always waste products, the composition of which is subject only to applicable environmetal guidelines.
In a compressor, especially a rotary screw compressor, some of the lubricant becomes entrained in the air being compressed and is discharged from the compressor along with the compressed air stream. Unlike the discharge product of an engine, however, the discharge product of a compressor is usually a product intended for a specific use, and its composition is generally required to be carefully controlled; it must also be relatively free of oil. It is, therefore, necessary to remove the oil from the air stream. Once the oil is removed from the air stream, it becomes advantageous to recover it and recycle it back to the compressor.
The compressor lubricant is thus continuously recovered from the air stream and recycled back to the compressor where it is subjected to the hostile environment within the compressor on a repetitive basis.
Since compressor lubricants are subject to such multiple exposure to severe operating conditions, oxidative stability becomes one of the most important properties a compressor lubricant must have. This property, in fact, is one of the main factors which determine the useful serve life of a compressor lubricant.
In addition to its effect on the useful service life of the lubricant itself, the oxidative stability of the lubricant also affects the performance of the compressor equipment. One of the most difficult lubricant related problems encountered in compressor equipment is the formation of carbon deposits within the compressor and associated piping. This is caused by oxidation of lubricant contained in the air stream as it passes through the equipment. This particular problem is one of the major maintenance items which require periodic shutdown of compressor systems.
While there are many different types of lubricant compositions known for use in compressors, each of them is deficient in oxidative stability or some other important property.
Mineral oils, for example, are known to be excellent lubricants. Unfortunately, however, the viscosity/temperature relationships of mineral oils are such that at extremely hot temperatures these oils are too thin and at extremely cold temperatures they are too thick; pour points are not low enough. Even those mineral oils containing added viscosity index improvers, pour point depressors, and other additives are not completely satisfactory because they have relatively high volatilities, low flash points, poor thermal and oxidative stabilities, and tend to form carbon and sludge deposits.
Silicone oils offer a number of advantages including good viscosity versus temperature performance, good thermal stability, oxidative stability, and relatively low volatility. The silicones, however, are expensive and are known to be somewhat deficient in their ability to provide lubricity for metal to metal contact. It appears that lubricity of the silicones can be improved through the use of certain additives and that some new silicone compounds have been synthesized which provide good lubricity characteristics. Although they have been used in some compressor applications, the silicones are not being widely used; their high cost and reputation for poor lubricity being discouraging factors.
The silicate esters have excellent viscosity versus temperature characteristics, good thermal stability, wide operating temperature ranges, and low volatility. Their oxidative stabilities, however, are only fair, and these compounds are very unstable in the presence of moisture.
The phosphate esters are relatively good lubricants, are fire resistant, and have low vapor pressures. These fluids are, therefore, attractive candidates for high temperature lubricant applications. Unfortunately, however, the viscosity indices of phosphate esters are relatively poor, and this limits their operating temperature range. In addition, the phosphate esters are expensive and require special sealing materials.
Several diester-based lubricants (i.e., esters of dicarboxylic acids) are now commercially available. These diester fluids are relatively good lubricants and exhibit improved service life in compressors as compared to conventional mineral oil lubricants. The diesters offer relatively high thermal stabilities, wide operating temperature ranges, low pour points, and relatively high flash points. However, even though the oxidative stability of these compounds is better than that of most of the other prior art compressor lubricants, it is still a limiting factor on the service life of the lubricant.
Polyol esters such as carboxylic acid esters of pentaerythritol, dipentaerythritol, or trimethylolpropane, although known to be lubricants which are characterized as having high oxidative stabilities, are not known to be useful as compressor lubricants.
Therefore, a need exists for a method of lubricating a compressor wherein a lubricant capable of extended service-life is used and the incidence of lubricant-related maintenance problems is reduced.